In perforating sheet material, a two-dimensional hole matrix is frequently sought with rigorous limits on perforation spacing uniformity as between rows and columns of the matrix. An illustrative field of current interest is that of perforating cigarette filter tipping paper, where hole matrix uniformity enables consistency of cigarette dilution characteristics. In various known mechanical puncture and electric arc perforating practices, row spacing is rendered precise by providing an individual perforating device for each row. Uniformity in the spacing of perforations made in each row, and hence precise column spacing is achieved by synchronizing operation of each perforating device. Since the perforating devices, e.g., pin or electrode pair, are physically limited in size, these practices can readily accommodate quite close spacing of adjacent rows of the matrix.
The prior art has also encompassed perforating practices involving lasers providing pulsed or continuous light energy in row-column perforation. In these efforts, however, there generally has been an apparent preference, for economic and physical size reasons, for use of a single laser serving both row and column perforation. Known single laser practices of type affording spacing uniformity have involved the splitting of the laser beam into plural beams, one for each row, and the focusing of light onto a sheet member by use of an individual lens for each row. Spacing of perforations by precise limits within each row has been sought by inclusion of a movable reflective element in each of the plural beam paths. Complexity attends precision movement, e.g., vibration or pivoting, of such reflective element into and out of its reference plane, to uniformly locate holes in rows, and the present state of the art is accordingly limited.
The foregoing prior art practices and references illustrating same and other practices are further discussed in the statement filed herein pursuant to 37 CFR 1.97 and 1.98.